Such an antenna is disclosed in numerous patent publications of the assignee, including U.S. Pat. Nos. 5,854,608, 5,945,963 and 5,859,621. These patents disclose antennas each having one or two pairs of diametrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core of a material having a relative dielectric constant greater than 5, with the material of the core occupying the major part of the volume defined by the core outer surface. A feed structure extends axially through the core, and a trap in the form of a conductive sleeve encircles part of the core and connects to the feed structure at one end of the core. At the other end of the core, the antenna elements are each connected to the feed structure. Each of the antenna elements terminates on the rim of the sleeve and each follows a respective longitudinally extending path. In the antenna disclosed in the assignee's U.S. Pat. No. 6,369,776, the feed structure, which is a coaxial transmission line, is housed in an axial passage through the core, the diameter of which passage is greater than the outer diameter of the coaxial line. The outer shield conductor of the coaxial line is thereby spaced from the wall of the passage. In practice, the coaxial line is surrounded by a plastics tube which fills the space between the outer shield conductor and the wall of the passage and has a relative dielectric constant between that of air and that of the material of the core.
The conductive sleeve referred to above is coupled to the outer shield of the feed structure where it emerges at a proximal end face of the antenna to form a balun at the frequencies of certain modes of resonance of the antenna. This effect occurs when the electrical length of the sleeve and its connection to the feed structure (with respect to currents on the inner surface of the sleeve) is nλg/4 where λg is the guide wavelength of the relevant resonance.
Dielectrically-loaded antennas such as those described above can be used for the reception of circularly polarised signals transmitted by satellites, such as GPS navigation signals, satellite telephone signals and broadcast signals. The antennas also have applications in the field of mobile telephones, e.g. cellular telephones, and well as wireless local area networks.